The present invention relates to encoders and more particularly to rotational encoders which are useful in closed loop step motor control systems.
In recent years, there has been an increasing demand for high speed step motors in technologies such as high speed printers where step motors are used both in the escapement and character selection apparatus. Open loop control of step motors has the disadvantage that the motor may not be able to follow the input pulse train with sufficient accuracy that the maximum speed which the motor can be operated at is limited. In addition, maximum efficiency cannot be achieved with open loop control. Closed loop step motor systems are directed at overcoming these disadvantages by using positional feedback to the step motor to determine the proper rotor position at which phase switchings of the motor may occur. Thus, closed loop controlled step motor systems appear to be the approach that the field is taking at the present time towards the achievement of higher speed, stability and efficiency in step motor operations. Conventionally, the feedback sensors used in closed loop step motor control systems are optical devices or magnetic devices which provide pulses proportional to the rotation of the step motor.
Conventional encoders used to provide the requisite step motor system feedback may be slotted disks mounted on the shaft of the step motor so as to rotate in synchronization with step motor. Associated with the slotted disk are sensing means comprising a source of light on one side of the disk and a photosensor on the other side for detecting the absence or presence of a slot through the sensing of transmitted light. Of course, in such step motor systems, the position of the photosensor and consequently the timing of the sense pulse with respect to the step position is a very critical aspect of feedback control of the step motor. The slots in the encoder disk are conventionally uniformly spaced and equal in number to the number of steps per revolution of the motor. Consequently, each time a slot passes through the beam from the light source to the photosensor, a pulse is generated by the photosensor and fed back to the control system of the step motor.
In the operation of such encoders, the sensing means are adjusted to a fixed angle which may be referred to as the "lead angle" or the "switching angle" with respect to the detent or equilibrium position of the motor. The lead angle is in effect the number of degrees in advance of (in lead of) a given detent position from which the corresponding phase is sensed and consequently switched. Consequently, the speed of a step motor is directly related to the lead angle.
In the case of some encoders having slots or other rectilinear indicia, the lead angle may be set by adjusting the position of the sensing means or the indicia being sensed with respect to a given motor detent position. In cases where the step motor travels in only one direction, the means for adjusting the position of the sensing means with respect to the leading edges of the slot are relatively straight forward. Usually a single sensing means may be utilized. On the other hand, if the step motor is to be a bidirectional step motor and particularly high speed bidirectional step motor which requires accurate lead angle adjustments in both directions, the problem of lead angle adjustment becomes more complex. It should be noted that where the closed loop feedback system is a common one pulse per step system, i.e., the encoder disk has only one slot per step, it is still very difficult and less than fully accurate to optimize a bidirectional closed loop system. As will be described hereinafter in greater detail with respect to the drawings of the prior art, this is accomplished by positioning each encoder slot at the center equidistant from its two adjacent motor detent positions. This provides an equal lead angle for either direction equivalent to half a step provided the sensing indicia is of negligible width.
However, where the lead angle in either or both directions is to be some value other than a half step, this single sensing means apparatus will be substantially inoperative.
A standard approach in the present technology towards the solution of the above problem is to use dual sensing means, i.e., a first sensor positioned to monitor encoder disk when the step motor shaft is rotating in one direction and a second sensor for monitoring the encoder disk when it is rotating in the reverse direction. However, this approach requires additional equipment and is more complex and expensive than monitoring by a single sensing means.
Similar problems are encountered with linear step motors when such linear step motors are operated in the bidirectional mode.